1. Field of the Invention
The present invention relates to molded plastic articles and methods for molding plastic articles, and more particularly to an apparatus and method for forming a one-piece, hollow, blow molded article such as a tank liner including an insert integrally molded with the article.
2. Discussion of the Related Art
An insert or fixture often must be secured to a blow molded article in order to enable the article to be utilized in the desired manner. For instance, the insert may be stronger, be better able to hold dimensional tolerances, or have other physical characteristics that are superior to those of the blow molded article for a particular purpose. The article is often blow molded onto a previously-formed insert to negate the need to bond the insert to a preformed article.
For instance, a water softener resin tank liner is often blow molded onto a premanufactured injection molded threaded insert. The insert serves as a mounting point for the water softener's control valve after the tank liner is reinforced with a fiberglass wrap to form a completed resin tank. In one such process, a threaded insert is screwed onto a mandrel that is capable of positioning the insert within a mold cavity in which the tank liner is formed. In this method, the insert is first positioned on the mandrel, which is then moved to position the insert at a desired location within the mold cavity. Plastic material is then blown into the cavity in a conventional blow molding process to cause the plastic material to conform to the shape of the mold cavity and surround the insert to form the tank liner. As the plastic material cools around the insert, the insert becomes fixed to the tank liner. When the tank liner is sufficiently cool, the mandrel is disengaged from the insert and withdrawn from the mold such that the tank liner and insert can be removed from within the mold cavity and the process can begin again to form a second tank liner with another insert.
When blow molding a water softener resin tank liner about a threaded insert, the mandrel must also include a correspondingly threaded exterior surface in order to securely hold the insert on the mandrel before and during the molding process, and to enable the mandrel to easily engage the insert without damaging the threads on the softer insert. When inserts and mandrels having threaded surfaces are utilized in processes of this type, after the tank liner has been formed around the insert within the mold cavity, the mandrel is rotated and lowered simultaneously to disengage the mandrel from the insert. This synchronization is difficult to achieve and maintain on a production basis and is usually synchronized with the mold opening process. The mandrel is rotated and lowered at a very low speed to avoid damaging the threads on the insert. However, based on the precise synchronization required for the speeds of rotation and lowering of the mandrel in conjunction with part removal, and on the normal variances in the manufacture of the threads on the inserts used, often times the mandrel damages the threads on the insert.
Care must also be taken to assure that the mandrel seals against the insert in a manner that prevents plastic from the parison from seeping into the insert from below and ruining the threads. This seal can be achieved using a threaded mandrel, but forming such a seal is hindered by the fact that some tolerance must be provided between the mandrel and the threads of the insert to accommodate non-uniformities in size and/or diameter of the mandrel and/or insert.
In addition to the problems associated with the presence of the threads on the interior of the insert by the mandrel, the cycle time requirements for properly attaching the insert on the mandrel prior to placement of the insert within the mold cavity also present a significant limitation to the process of forming a tank liner with an insert in this manner. Specifically, because the insert and mandrel must be initially threadedly engaged with one another to securely position and hold the insert within the mold cavity, a significant amount of time is required to properly engage the insert with the mandrel. In addition, the slow speeds required when withdrawing the mandrel from the insert to avoid damaging the threads on the insert further increase the overall cycle time for this process.
Therefore, it is desirable to develop an apparatus and method for supporting an insert within a mold cavity used to form a tank liner or other blow molded article that does not damage the insert while moving and/or holding the insert within the mold cavity or when disengaging from the insert after formation of the article. It is also desirable to develop an apparatus and method that can greatly decrease the cycle time for forming the article by decreasing the overall time required for engaging a mandrel with an insert, moving the mandrel to position the insert within the mold cavity, and withdrawing the mandrel from the insert after formation of the article within the cavity.